[llvm] [RISCV][TTI]Use processShuffleMasks for cost estimations/actual per-register shuffles (PR #118103)

[Philip Reames via llvm-commits]

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@@ -389,7 +389,103 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
   // First, handle cases where having a fixed length vector enables us to
   // give a more accurate cost than falling back to generic scalable codegen.
   // TODO: Each of these cases hints at a modeling gap around scalable vectors.
-  if (isa<FixedVectorType>(Tp)) {
+  if (ST->hasVInstructions() && isa<FixedVectorType>(Tp)) {
+    MVT LegalVT = LT.second;
+    InstructionCost NumOfDests = InstructionCost::getInvalid();
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preames wrote:

For readability, can you extract this into a helper function called costShuffleViaVRegSplitting?  That would make it easy to map the lowering code, and would allow use of early return to reduce indentation.  

https://github.com/llvm/llvm-project/pull/118103